*******Note that forecast and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.*********
...WEDNESDAY SEPTEMBER 25 2024 2:43 PM EDT...
Releasing my updated track and intensity forecast for tropical cyclone Helene now due to the large swath of land areas that this system is expected to impact... which shows a west shift in the track and a higher intensity forecast than I previously showed. My full update which includes information on current disturbances across rest of the Atlantic tropics will be released later this evening.
TROPICAL STORM HELENE (RECENTLY UPGRADED TO HURRICANE HELENE)... Visible satellite image of recently-upgraded Hurricane Helene as of 1536Z:
As documented in special update #88B (https://www.infohurricanes.com/post/my-2024-atlantic-hurricane-season-birdseye-view-post-88b-special-update)... tropical storm Helene's track took a west hook toward the northeastern Yucatan peninsula coast while its center was likely cyclonically swinging around the remainder southern portion of the broad tropical low pressure field it formed within. During this time the adjacent Yucatan peninsula upper vorticity shifted west and away toward the eastern Bay of Campeche while likely shoved by the previous southeastern US upper ridge wave... which helped relax the shear over Helene such that it developed a more symmetric core of thunderstorms covering its center that allowed it to strengthen a little quicker than my previous intensity forecast showed. The latent heat release of the strong thunderstorm core may have also helped in shoving the adjacent cool-core upper vorticity into the eastern Bay of Campeche in recent hours. From 8 AM to 11 AM EDT the track has returned to the previously expected north turn which has just kept the center from making landfall over the northeastern Yucatan peninsula... and during this time Helene strengthened further to a category 1 hurricane with 80 mph maximum sustained winds. Recently (around 2 PM EDT) the thunderstorm core has relaxed in intensity on colorized infrared satellite pictures... perhaps while the strengthening circulation inhaled some of the dry sinking air associated with the western convergence zone of the adjacent eastern Bay of Campeche upper vorticity. Likewise the NHC update at 2 PM EDT shows that Helene has not strengthened between 11 AM EDT and 2 PM EDT.
The track forecast still calls for a north-northeast acceleration across the eastern Gulf of Mexico and into the southeastern United States from northern Florida as the current eastern North America upper trough and eastern Bay of Campeche upper vorticity merge into a lengthy upper trough that steers Helene. The track forecast is nudged westward to account for Helene's prior westward swing toward the Yucatan peninsula noted in the prior paragraph and now calls for the center to make landfall over the far eastern Florida panhandle instead of the northwestern part of the Florida peninsula. After landfall... the south part of the steering upper trough is forecast to become a cut-off upper vortex to the east of an amplified upper ridge... with Helene arcing north then eventually north-northwest around the east side of the upper vortex... a path that takes the inland remnant system across the eastern US. During this time Helene transitions into a remnant frontal cyclone supported by the eastern divergence zone of the steering upper vortex... and likewise pulls cold air associated with the upper vortex toward itself. Because the intensity forecast has trended stronger as noted below... Helene will be able to pull the cold core air associated with the upper vortex closer to the warm air it was previously under... resulting in a stronger steering upper jet in the air mass contrast zone. This is why the center of ex-Helene is now forecast to reach the vicinity of the southwestern North Carolina border sooner than I previously showed. Regarding intensity... I have increased the forecast and now show a top-end category 2 for landfall based on Helene strengthening a little faster than the prior forecast... however I still trend below the latest NHC intensity forecast as of this writing (which calls for a category 3 peak) due to the recent pause in strengthening noted in the prior paragraph. There are two scenarios possible regarding intensity up until landfall... either the southwesterly wind shear associated with the steering upper trough keeps Helene on the weaker side... or Helene re-develops another strong thunderstorm core whose latent heat release can develop a local warm core upper anticyclone that deflects the shear and allows the southwesterly upper flow to enhance the northern outflow of the anticyclone which could result in a surprise amount of intensification. I see my current intensity forecast as the middle ground between the two possibilities. Note that Helene is a larger sized storm as it was born within a broad low pressure field... and will continue to remain large due to the size of the eastern divergence zone of the steering upper trough/vortex. The large wind field will drive a life-threatening widespread coastal storm surge toward the landfall region regardless of the exact category number assigned to Helene's peak intensity. Final note regarding intensity... the post-landfall decay rate of Helene could be slower than normal due to the aforementioned field of upper divergence.
Regarding impact to land areas:
(1) For the northeastern Yucatan peninsula... interests should remain sheltered in place until the gusty winds and coastal surf relax as Helene pulls away later today. Heavy rainfall with flash flooding remains a concern for today.
(2) For the Cayman Islands… periods heavy rainfall will continue from the southeastern outer bands of Helene over the next several hours.
(3) For western Cuba… heavy rainfall... gusty tropical storm force winds... and coastal surf are upon the area. Weather conditions will improve gradually over the next 24 hours once Helene accelerates north-northeast and away
(4) For the Florida Keys... expect periods of heavy rainfall from Helene's outer eastern rain bands and coastal surf over the next 36 hours. The west shift in the forecast track has reduced the potential for gusty tropical storm force winds here... however tropical storm force gusts cannot be ruled out for Key West.
(5) The southwest Florida peninsula coast will be subject to coastal surf by tomorrow. For the remainder of the Florida peninsula west coast... Florida panhandle... and Alabama coast... coastal surf arrives by late tomorrow and early Friday… becoming life-threatening for the eastern Florida panhandle and northwestern Florida peninsula no later than early Friday (listen to the advice of local government officials and news media... particularly evacuation orders). Damaging hurricane to tropical storm force winds are expected to spread onshore across the northern half of Florida peninsula... eastern half of the Florida panhandle... and southern Georgia no later than early Friday… with the most severe wind toward the eastern Florida panhandle coast and/or far northwestern Florida peninsula coast. Heavy rainfall with flooding potential is expected to spread across central and northern Florida… then across much of the southeast US Friday through Saturday. Due to the slowed inland decay rate mentioned at the end of the prior paragraph… gusty winds are also possible across much of the southeast US Friday through Saturday. The inland southeastern US region that maybe subject to gusty winds (with scattered damage) and heavy rainfall potential includes Georgia... Alabama... Mississippi... eastern Arkansas... southeastern Missouri... Tennessee... Kentucky... southern Illinois... southern Indiana... southern Ohio... the western Virginias (including most of West Virginia)... and the western Carolinas.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 25)... 80 mph maximum sustained wind hurricane centered just offshore of the northeastern Yucatan peninsula at 21.6N-86.3W
IOH 24 Hr Forecast (1200Z Sep 26)... 100 mph maximum sustained wind hurricane in the eastern Gulf of Mexico at 25.5N-85.5W
IOH 36 Hr Forecast (0000Z Sep 27)... 110 mph maximum sustained wind hurricane centered over the far eastern Florida panhandle coast at 29.8N-83.5W
IOH 48 Hr Forecast (1200Z Sep 27)... Remnant frontal cyclone centered over the NC/ SC/ GA border intersection at 35N-83W
*****National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT*************************
Landfall (0000Z Sep 27)… 125 mph maximum sustained wind major hurricane centered over the eastern Florida panhandle coast at 29.7N-84.6W
4-Day Position (1200Z Sep 29)… 15 mph maximum sustained wind dissipating remnant low centered over the western Kentucky/Tennesee border at 36.6N-87.9W
Comments